// Source : https://leetcode-cn.com/problems/remove-k-digits/
// Author : jkbs487
// Date   : 2020-12-16

/***************************************************************************************************** 
 *
 * Given a non-negative integer num represented as a string, remove k digits from the number so that 
 * the new number is the smallest possible.
 * 
 * Note:
 * 
 * The length of num is less than 10002 and will be &ge; k.
 * The given num does not contain any leading zero.
 * 
 * Example 1:
 * 
 * Input: num = "1432219", k = 3
 * Output: "1219"
 * Explanation: Remove the three digits 4, 3, and 2 to form the new number 1219 which is the smallest.
 * 
 * Example 2:
 * 
 * Input: num = "10200", k = 1
 * Output: "200"
 * Explanation: Remove the leading 1 and the number is 200. Note that the output must not contain 
 * leading zeroes.
 * 
 * Example 3:
 * 
 * Input: num = "10", k = 2
 * Output: "0"
 * Explanation: Remove all the digits from the number and it is left with nothing which is 0.
 * 
 ******************************************************************************************************/

class Solution {
public:
    string removeKdigits(string num, int k) {
        string ans;
        if(k == 0) return num;
        deque<char> dq;
        for(auto n : num) {
            while(k-- > 0 && !dq.empty() && dq.back() > n) {
                dq.pop_back();
                k--;
            }
            dq.push_back(n);
        }
        while(k-- > 0) dq.pop_back();
        while(!dq.empty() && dq.front() == '0') dq.pop_front();
        if(dq.size() == 0) dq.push_back('0');
        ans.insert(ans.begin(), dq.begin(), dq.end());
        return ans;
    }
};
